The problem of protecting workstations against contamination from the ambient air is encountered in various branches of industry, in particular in the food industry, in the food-preparing industry and trade (e.g. butchers and caterers, for example), in the pharmaceutical, chemical, microbiology, and aircraft industries, in hospital and surgical fields, in laboratories, and in military applications, or even domestic applications.
The invention applies to all fields in which the quality of the substance(s) processed or treated at a workstation must be preserved or maintained, whatever the pollution in the surrounding atmosphere.
In general, in order to protect a work surface, the work surface is installed inside a sterile "clean" room in which the air is filtered and treated so that it has a given level of cleanness.
The main drawback with that solution is that it is difficult to maintain air quality in a large volume under conditions that are economically acceptable. Another drawback is related to the fact that personnel must wear special clothing, which is itself sterile, which is difficult to maintain impeccably clean, and whose cleanness is difficult to monitor. Wearing such clothing is therefore tiresome. In addition, in spite of such special clothing, personnel and equipment present in the work room can pollute the air.
The close protection technique offers a solution to that problem.
Close protection consists in immersing the work surface and the objects carried thereby in a moving atmosphere of clean air which is continuously renewed.
The work surface is thus protected by a shield of clean air which shields the objects from external contamination, although the remainder of the room containing the work surface contains air that might be polluted. The personnel assigned to performing tasks on said objects can therefore be subjected to hygiene constraints that are less strict.
The work surface may be a stationary table top on which there are placed items that are to be processed or operated on either manually or automatically.
The work surface may also be a conveyor, e.g. a moving belt, which brings successive items to be processed to the work station and removes them once the processing operation is terminated.
By way of example, the items may be receptacles which are conveyed one behind the other by the moving belt, and which are filled by a dispensing head disposed in the protected zone, so that the filling substance is free from any contamination.
The relevant state of the art can be illustrated by Documents FR-A-1 257 562 and EP-A-0 375 343.
Document FR-A-1 257 562 concerns apparatus for maintaining an atmosphere that has determined characteristics in a chamber that is open on one side.
That chamber has a floor constituting a horizontal work surface over which a flow of clean air flows.
The chamber is open on one of its sides only, which side is accessible to an operator, and a second flow of clean air flows across the open side in the form of an approximately vertical curtain directed downwards from top to bottom.
The air is sucked in at the front edge of the work surface (the open side of the chamber), and it is recycled via atmosphere regeneration apparatus so that it flows around a closed-circuit path.
The horizontal flow of air that passes through the inside of the chamber has a speed distribution that is approximately constant over the entire height of the chamber. The value of the speed is independent of the height of the point in question relative to the work surface.
A major drawback with the apparatus described in FR-A-1 257 562 results from the fact the chamber is delimited on top by a ceiling, thereby considerably limiting the accessibility of the work surface.
That raises difficulties in particular when the processing operation to be performed on the items supported by or conveyed by said work surface requires machines or other equipment to be implemented above the work surface.
That is the case, for example, for dispensing and/or metering machines serving to fill receptacles conveyed by a moving belt for the purpose of filling them in the close protection zone.
The apparatus described in Document EP-A-0 375 343 includes a horizontal work surface which is unobstructed both from above and from one of its sides.
A fan disposed under the work surface, and having a filter disposed at its outlet, generates air under pressure in a box opening into a dispensing chamber situated on the non-open side of the work surface.
This chamber produces two flows, namely a horizontal flow passing over the work surface, and a multi-directional flow made up of a plurality of small jets produced by holes formed in the various partitions of the chamber.
The inside of the chamber is provided with cylindrically arcuate guides deflecting the air coming from the box so as to produce the above-mentioned horizontal flow.
Although that known apparatus offers the advantage of being accessible both from one of the sides of the work surface and from above, it nevertheless suffers from the drawback that the speed distribution in the horizontal flow over the work surface is constant.
Unfortunately, distribution of that type does not fully protect the items placed on the work surface. Tests performed by the Applicants have shown that it is desirable to provide a speed gradient, with speeds increasing with increasing distance from the work surface. Particularly effective protection is obtained when the clean air moves at a low speed, or is even almost stationary, in the vicinity of the work surface, while the upper layer of the flow has a relatively high speed.